Make The Dough. In a small bowl, heat milk in a microwave until melted and lukewarm to the touch (about 110°F/45°C). Sprinkle the yeast over the lukewarm milk mixture and let it sit for 10 minutes until it starts to get foamy on top.
In a bowl of a standing mixer fitted with a dough hook, add the melted butter, egg, sugar and salt and mix. Add 3 cups of flour and salt then yeast mixture and mix on low speed. You can do this step by hand with a wooden spoon. Add remaining flour, 1 tablespoon at a time and stir on medium-low speed until the dough comes together and pulls away from the sides of the bowl. Dough should form into a nice ball and be slightly sticky. If it’s too sticky, add 1 tablespoon of flour one at a time until it reaches the consistency. Make sure that you use at around 4 cups at maximum. Increase to medium speed and keep kneading for 5-6 minutes or knead by hands on a floured surface for 8-10 minutes.
Transfer dough to a lightly greased bowl (use a few drops of oil or a non-stick spray) and lightly grease the top of the dough. Cover the dough with plastic wrap and let it rest in a warm place for about 1 hour or 1 1/2 hours until it doubles in size. Resting time depends on the humidity and temperature in where you live.
Make the filling. While dough is resting, prepare the filling. Combine softened butter and pistachio butter in a small bowl. In another bowl mix in sugar, cocoa powder and salt.
After dough has doubled in size, transfer dough to a floured surface and roll out into a 14 x 9 rectangle. Spread the butter mixture over the dough, the sugar chocolate mixture then a layer of chopped chocolate and press gently, leaving a small margin around the edges. Tightly roll the dough up to form a log, starting from the longer side and place seam side down. Then use a sharp knife or a dentist floss to cut the dough evenly into 10-12 rolls (around 1inch section) then place into a prepared pan (You can use a 9x9 inch square or round pan). Cover and let it rise again for about 30-45 minutes.
Preheat oven to 350ºF/180ºC. Remove cover and bake the rolls for 20-25 minutes until the tops are browned and a toothpick inserted into the rolls comes out clean. Allow them to cool for 10-15 minutes before adding glaze.
Make Chocolate Ganache. Place chopped chocolate in a medium sized bowl and set aside. In a small saucepan bring cream to a simmer and bubbles are just starting to form around the edges of the pan. Remove from the heat. Pour over chocolate, allow to sit for 3-5 minutes. With a whisker or a metal spoon, gently stir in the cream and chocolate together until a smooth, silky consistency.
Spoon frosting on top of the rolls when they are still warm, sprinkle some chopped pistachios (optional) and serve immediately.
The rolls can be kept fresh and soft in an airtight container in the refrigerator for up to 3 days. Reheat leftovers in the microwave for about 20 seconds before serving.